1. Field of the Invention.
This invention relates to liquid chromatographic detectors and more particularly to improved flow cell constructions for liquid chromatography.
2. Prior Art.
A chromatographic detector is a device which supplies an output signal related to the amount or rate of change of the amount of a sample to be detected in the effluent of a chromatographic column. It indicates elution of the separated components of the input substance by the column and provides a measure of the amount of each component. It is usually the most sophisticated and one of the most expensive components in a chromatograph. The majority of high performance liquid chromatograph detectors in use today are UV or visible light absorption and refractive index detectors. Light rays are directed through the sample and the effect of the sample on the rays, e.g., light absorption, is detected by a photocell.
Good detectability, i.e., ability to detect a small sample, is desired so that small samples and small column capacity can be used, resulting in shorter analysis times. Some new, highly efficient, column packing materials have inherent low capacity and require detectors capable of detecting small samples. Also, low sample solubility in the mobile phase may limit the amount of sample available for detection.
With small flow cell volumes, present flow cell geometries offer low light transmission, poor flow geometry, and become expensive to manufacture. Parallel light rays are used with flat or cylindrical cell entry windows. Also, convergent light rays have been used with flat windows, and typically the convergent light rays are focused on the entry window for maximum transmission to the cell. The path lengths of parallel light rays passing through a cylindrical cell differ across the width of the cell. Also, the path lengths of convergent light rays passing through a flow cell with flat entrance and exit windows differ across the area of the flow cell. Since light absorbance is a function of the path length through the cell, the output signal from such cell is nonlinear; that is, the detector response does not change linearly with the amount of sample present. Linearity is desirable because it facilitates quantitation of the results, eliminating the need of functional callibration curves to determine the quantity of the component present.
In particular, there is a need in the prior art for a high light transmission cell with a small aperture to accommodate a small sample for use in high performance liquid chromatography and which affords equal path length for the light rays directed through the cell.